Metabolic chamber of secrets: effects of protein on metabolism when overeating

Protein. Carbs. Fat. Each of these three macronutrients have their aficionados in the wild world of Internet nutrition discussion. There is some evidence that protein consumption is the key to diet-influenced changes in metabolic rate, and there are many theoretical mechanisms that have been proposed. Unfortunately, it’s usually much easier to propose mechanisms to explain correlations than it is to actually test them.

Studying the effect of diet on metabolic rate is brutally difficult because of how challenging it is to control all of the variables associated with energy expenditure and metabolism. This is in addition to all of the usual challenges associated with nutritional trials, such as compliance, accurate reporting, and regular assessment. In fact, one of the only ways to reliably study metabolism is to literally lock a person in a room and measure everything that goes into and comes out of that room over a specific time frame, including exhaled air and sometimes even urine and feces. Figure 1 compares indirect calorimetry (which was done in this study) to direct calorimetry. Both are ways to estimate heat produced by living organisms, and hence figure out energy expenditure. The accuracy of these two methods (especially in light of the challenges of performing direct calorimetry) is an active area of debate^[1].

Figure 1: Indirect versus direct calorimetry

This trial used that kind of “metabolic chamber” during certain points in the study to assess whether protein consumption affects changes in energy expenditure in people eating a caloric surplus. This caloric surplus was an important aspect of the study because protein metabolism is somewhat dependent on the amount of calories consumed^[2], though it does affect the applicability of the study to dieting strategies. Although this study isn’t directly applicable to popular diets (such as low carb and even ketogenic dieting) because of the calorie and specifically carb surplus, it does seek to understand the fundamental effects of high protein consumption on metabolic rates and other parameters associated with metabolism, such as muscle mass gain. It does directly address a question that sometimes pops up: why do certain people seem to overeat but not gain as much weight as you’d expect?

High-protein diets are thought to increase metabolism, although the mechanisms aren’t fully fleshed out, and studying the hypothesis is difficult because of how many variables must be controlled. The purpose of this study was to examine whether protein intake affects metabolic rates under a calorie surplus in tightly-controlled conditions.